Electrolyte solution for lithium-sulfur battery and lithium-sulfur battery comprising same

ABSTRACT

The liquid electrolyte for a lithium-sulfur battery according to the present invention exhibits an excellent sulfur utilization rate when used in a lithium-sulfur battery, and exhibits excellent stability. Accordingly, the liquid electrolyte for a lithium-sulfur battery according to the present invention is capable of enhancing a life time property while securing a capacity property of a lithium-sulfur battery.

TECHNICAL FIELD

This application claims priority to and the benefits of Korean PatentApplication No. 10-2016-0080646, filed with the Korean IntellectualProperty Office on Jun. 28, 2016, and Korean Patent Application No.10-2017-0028616, filed with the Korean Intellectual Property Office onMar. 7, 2017, the entire contents of which are incorporated herein byreference.

The present invention relates to a ternary liquid electrolyte for alithium-sulfur battery and a lithium-sulfur battery including the same.

BACKGROUND ART

With recent development of portable electronic devices, electricvehicles and large capacity power storage systems, demands for largecapacity batteries have arisen. A lithium-sulfur battery is a secondarybattery using a sulfur series material having sulfur-sulfur bonds (S—Sbonds) as a positive electrode active material and using lithium metalas a negative electrode active material, and sulfur, a main material ofa positive electrode active material, has advantages of being veryabundant in resources, having no toxicity and having a low atomicweight.

In addition, a lithium-sulfur battery has theoretical discharge capacityof 1672 mAh/g-sulfur and theoretical energy density of 2,600 Wh/kg,which is very high compared to theoretical energy density of otherbattery systems currently studied (Ni—MH battery: 450 Wh/kg, Li—FeSbattery: 480 Wh/kg, Li—MnO₂ battery: 1,000 Wh/kg, Na—S battery: 800Wh/kg), and therefore, has received attention as a battery having a highenergy density property.

However, a lithium-sulfur battery has not been commercialized so far.This is due to the fact that, when using sulfur as an active material,the percentage of the sulfur used in an electrochemical reaction (sulfurutilization rate) is low and sufficient capacity as theoretical capacityis not secured. In order to overcome such a problem, positive electrodematerials having increased sulfur impregnation, liquid electrolytescapable of increasing a sulfur utilization rate, and the like have beendeveloped.

As a liquid electrolyte solvent of a lithium-sulfur battery,1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) having an excellentsulfur utilization rate have currently been used most often. These areused either alone or as a mixture, and Korean Patent ApplicationLaid-Open Publication No. 10-2009-0086575 discloses a lithium-sulfurbattery having 1,3-dioxolane in separated a negative electrode and1,2-dimethoxyethane in a positive electrode so as to be presentnon-uniformly using a polymer.

However, the solvent has a disadvantage of being readily decomposedduring battery driving. When the solvent is decomposed, gas such ashydrogen, methane and ethene is generated producing a swellingphenomenon, and as a result, battery life-shortening is caused.

Accordingly, in order to obtain a stable life time property in alithium-sulfur battery, development of stable liquid electrolytes thatdo not cause decomposition during battery driving has been required.

PRIOR ART DOCUMENTS

Korean Patent Application Laid-Open Publication No. 10-2009-0086575,Separation of Electrolyte

DISCLOSURE Technical Problem

In view of the above, the inventors of the present invention havestudied a liquid electrolyte solvent composition of a lithium-sulfurbattery, and as a result, have completed the present invention.

Accordingly, an aspect of the present invention provides a liquidelectrolyte for a lithium-sulfur battery having excellent stability.

Another aspect of the present invention provides a lithium-sulfurbattery including the liquid electrolyte.

Technical Solution

According to an aspect of the present invention, there is provided aliquid electrolyte for a lithium-sulfur battery comprising:

a lithium salt; and

a non-aqueous solvent,

wherein the non-aqueous solvent includes

i) cyclic ether including two oxygens in a ring structure;

ii) glycol ether represented by the following Chemical Formula 1; and

iii) linear ether represented by the following Chemical Formula 2:R¹—O—(CH₂CH₂O)_(x)—R²   [Chemical Formula 1]R³—O—(CH₂CH₂O)_(y)—R⁴   [Chemical Formula 2]

(In Chemical Formulae 1 and 2, R¹ to R⁴, x and y are the same asdescribed in the specification.)

Herein, the cyclic ether may be 5-membered to 7-membered cyclic etherunsubstituted or substituted with a C1 to C4 alkyl group or alkoxygroup.

Herein, the cyclic ether may be dioxolane or dioxane unsubstituted orsubstituted with a C1 to C4 alkyl group or alkoxy group, and specificexamples of the cyclic ether may be one type selected from the groupconsisting of 1,3-dioxolane, 4,5-diethyl-1,3-dioxolane,4,5-dimethyl-1,3-dioxolane, 4-methyl-1,3-dioxolane,4-ethyl-1,3-dioxolane, 1,3-dioxane, 1,4-dioxane, 4-methyl-1,3-dioxaneand 2-methyl-1,3-dioxane.

Herein, the glycol ether may be one type selected from the groupconsisting of 1,2-dimethoxyethane, ethylene glycol ethylmethyl ether,diethylene glycol dimethyl ether, triethylene glycol dimethyl ether andtetraethylene glycol dimethyl ether.

Herein, the linear ether may be one type selected from the groupconsisting of ethylene glycol ethylmethyl ether, ethylene glycol diethylether, dipropyl ether, diisopropyl ether, dibutyl ether and diisobutylether.

Herein, the cyclic ether may be included in 10% by volume to 40% byvolume of a total weight of the non-aqueous solvent.

Herein, the glycol ether and the linear ether may be included in avolume ratio of 1:3 to 3:1.

Herein, the lithium salt may be one type selected from the groupconsisting of LiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆,LiCF₃SO₃, LiCF₃CO₂, LiC₄BO₈, LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li,CF₃SO₃Li, (CF₃SO₂)₂NLi, (C₂F₅SO₂)₂NLi, (SO₂F)₂NLi, (CF₃SO₂)₃CLi,chloroborane lithium, lower aliphatic carboxylic acid lithium, lithiumtetraphenylborate, lithium imide and combinations thereof. The lithiumsalt may be included in a concentration of 0.1 M to 4.0 M.

Herein, the liquid electrolyte may further include an additive havingN—O bonds in a molecule. Specifically, the additive may be one or moretypes selected from the group consisting of lithium nitrate, potassiumnitrate, cesium nitrate, barium nitrate, ammonium nitrate, lithiumnitrite, potassium nitrite, cesium nitrite, ammonium nitrite, methylnitrate, dialkyl imidazolium nitrate, guanidine nitrate, imidazoliumnitrate, pyridinium nitrate, ethyl nitrite, propyl nitrite, butylnitrite, pentyl nitrite, octyl nitrite, nitromethane, nitropropane,nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine,dinitropyridine, nitrotoluene, dinitrotoluene, pyridine N-oxide,alkylpyridine N-oxide and tetramethylpiperidinyloxyl.

Herein, the additive may be included in 0.01% by weight to 10% by weightwith respect to 100% by weight of the liquid electrolyte.

According to another aspect of the present invention, there is provideda lithium-sulfur battery including the liquid electrolyte.

Advantageous Effects

A liquid electrolyte for a lithium-sulfur battery according to an aspectof the present invention exhibits an excellent sulfur utilization ratewhen used in a lithium-sulfur battery, and exhibits excellent stability.Accordingly, the liquid electrolyte for a lithium-sulfur batteryaccording to the present invention is capable of enhancing a life timeproperty while securing a capacity property of a lithium-sulfur battery.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing specific discharge capacity of batteries ofExamples 1, 2 and Comparative Example 1.

FIG. 2 is a graph showing specific discharge capacity of batteries ofExamples 3 to 6 and Comparative Example 2.

BEST MODE

Hereinafter, the present invention will be described in detail so thatthose skilled in the art may readily implement the present invention.However, the present invention may be implemented in various differentforms, and is not limited to the examples described herein.

Liquid Electrolyte for Lithium-Sulfur Battery

A solvent currently used most often as a liquid electrolyte solvent of alithium-sulfur battery is a mixed solvent of 1,3-dioxolane (DOL) and1,2-dimethoxyethane (DME). Using a mixed solvent of DOL and DME enhancesa sulfur utilization rate, and excellent results are obtained in termsof battery capacity.

However, using the combination in large batteries having high energydensity has a problem in that a life time property significantlydecline. As a result of experiments of the inventors of the presentinvention, it has been identified that, in a large capacity batteryusing a mixed solvent of DOL and DME, a capacity retention ratedecreases at a very high rate. In addition, the battery produces aconsiderable amount of gas while the solvent is decomposed duringdriving. Such a solvent decomposition phenomenon causes liquidelectrolyte depletion, and causes battery deformation such as batteryswelling and electrode deintercalation, and resultantly becomes a factorof battery life-shortening.

A liquid electrolyte according to the present invention includes cyclicether, glycol ether and linear ether, and exhibits excellent solventstability compared to existing liquid electrolytes, and exhibits anenhanced life time property.

Specifically, in order to improve a battery life-shortening caused byliquid electrolyte decomposition occurring when driving a lithium-sulfurbattery, the present invention provides a liquid electrolyte for alithium-sulfur battery including a lithium salt and a non-aqueoussolvent, wherein the non-aqueous solvent includes

i) cyclic ether including two oxygens in a ring structure;

ii) glycol ether represented by the following Chemical Formula 1; and

iii) linear ether represented by the following Chemical Formula 2:R¹—O—(CH₂CH₂O)_(x)—R²   [Chemical Formula 1]R³—O—(CH₂CH₂O)_(y)—R⁴   [Chemical Formula 2]

(In Chemical Formulae 1 and 2,

R¹ to R⁴ are the same as or different from each other, and eachindependently a C1 to C6 alkyl group, a C6 to C12 aryl group, or a C7 toC13 arylalkyl group,

x is an integer of 1 to 4,

y is an integer of 0 to 4, and

the ether of Chemical Formula 1 is different from the ether of ChemicalFormula 2.)

The C1 to C6 alkyl group mentioned in the present specification is alinear or branched alkyl group, and examples thereof may include amethyl group, an ethyl group, a propyl group, an isopropyl group, abutyl group, an isobutyl group, a sec-butyl group, a t-butyl group, apentyl group, a hexyl group or the like, but are not limited thereto.

Examples of the C6 to C12 aryl group mentioned in the presentspecification may include a phenyl group or a naphthyl groupunsubstituted or substituted with a C1 to C6 alkyl group.

Examples of the C7 to C13 arylalkyl group may include a benzyl group, aphenylethyl group, a phenylpropyl group or a phenylbutyl groupunsubstituted or substituted with a C1 to C6 alkyl group.

In Chemical Formula 1, R¹ and R² are the same as or different from eachother, and may preferably be a methyl group, an ethyl group, a propylgroup, an isopropyl group or a butyl group, and may more preferably be amethyl group, an ethyl group or a propyl group.

In Chemical Formula 2, R³ and R⁴ are the same as or different from eachother, and is preferably a methyl group, an ethyl group, a propyl group,an isopropyl group, a butyl group, an isobutyl group, a pentyl group, ahexyl group, a phenyl group or a benzyl group.

The liquid electrolyte according to the present invention includescyclic ether including two oxygens in the ring structure as a firstsolvent. The cyclic ether is 5-membered or higher cyclic etherunsubstituted or substituted with an alkyl group, and preferably5-membered to 7-membered cyclic ether unsubstituted or substituted witha C1 to C4 alkyl group or alkoxy group and more preferably dioxolane ordioxane unsubstituted or substituted with a C1 to C4 alkyl group oralkoxy group. Nonlimiting examples of the cyclic ether may include1,3-dioxolane, 1,3-dioxolane, 4,5-diethyl-dioxolane,4,5-dimethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane,1,3-dioxane, 1,4-dioxane, 4-methyl-1,3-dioxane, 2-methyl-1,3-dioxane,and the like, and preferably, 1,3-dioxolane may be used. The cyclicether has low viscosity and thereby has favorable ion mobility, and, dueto its high reduction stability, also exhibits high stability even whendriving a battery for a long period of time.

The first solvent is preferably included in 10% by volume to 40% byvolume with respect to the total volume of the non-aqueous solvent, andmore preferably included in 10% by volume to 30% by volume. When thefirst solvent is included in greater than the above-mentioned range,liquid electrolyte stability decreases making it difficult to secure aneffect of enhancing battery life.

The liquid electrolyte according to the present invention includes theglycol ether represented by Chemical Formula 1 as a second solvent.

Examples of the glycol ether include 1,2-dimethoxyethane, ethyleneglycol diethyl ether, ethylene glycol dipropyl ether, ethylene glycolethylmethyl ether, diethylene glycol dimethyl ether, diethylene glycoldiethyl ether, triethylene glycol dimethyl ether, triethylene glycoldiethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycoldiethyl ether and the like, and preferably include 1,2-dimethoxyethane,ethylene glycol ethylmethyl ether, diethylene glycol dimethyl ether,triethylene glycol dimethyl ether and tetraethylene glycol dimethylether. Such glycol ether may increase a sulfur utilization rate due toexcellent sulfur-based material solubility.

A third solvent of the liquid electrolyte according to the presentinvention may be, as the linear ether represented by Chemical Formula 2,glycol ether, or ether including one oxygen in the molecule such asabove. However, when the third solvent is glycol ether, this is acompound different from the second solvent.

Nonlimiting examples of the ether including one oxygen in the molecularstructure may include dimethyl ether, diethyl ether, dipropyl ether,diisopropyl ether, dibutyl ether, dipentylether, dihexyl ether,ethylmethyl ether, methylpropyl ether, butylmethyl ether, ethylpropylether, butylpropyl ether, phenylmethyl ether, diphenylether, dibenzylether and the like.

The third solvent preferably includes ethylene glycol ethylmethyl ether,ethylene glycol diethyl ether, dipropyl ether, diisopropyl ether,dibutyl ether, or diisobutyl ether. Such linear ether exhibits an effectof suppressing polysulfide dissolution and solvent decomposition, andthereby contributes to liquid electrolyte stability.

The 1,2-dimethoxyethane, the diethylene glycol dimethyl ether, thetriethylene glycol dimethyl ether, the tetraethylene glycol dimethylether and the like have excellent sulfur-based material solubility andincreases a sulfur utilization rate, and therefore, contributes toenhancing a battery capacity property. Meanwhile, the ethylene glycolethylmethyl ether, the ethylene glycol diethyl ether, the dipropylether, the diisopropyl ether, the dibutyl ether, the diisobutyl etherand the like have excellent stability, and decomposition hardly occursduring battery driving. Accordingly, proper mixing of these solvents hasan advantage of securing both a sulfur utilization rate and liquidelectrolyte stability.

The second solvent and the third solvent are preferably included in 60%by volume or greater with respect to the total volume of the non-aqueoussolvent. Herein, the relative ratio of the second solvent and the thirdsolvent may be properly adjusted depending on the types of electrodesused, battery capacity and the like, however, each is preferablyincluded in at least 10% by volume or greater with respect to the totalweight of the non-aqueous solvent in terms of securing battery capacityand stability. Specifically, the second solvent and the third solventare preferably mixed in a volume ratio of 1:3 to 3:1, and morepreferably mixed in a volume ratio of 1:2 to 2:1.

According to one preferred embodiment of the liquid electrolyte for alithium-sulfur battery according to the present invention, thenon-aqueous solvent of the liquid electrolyte includes 1-3 dioxolane asthe first solvent, 1,2-dimethoxyethane as the second solvent, andethylene glycol ethylmethyl ether or dipropyl ether as the thirdsolvent, and a volume ratio thereof may be from 1:1:1 to 1:2:2. This mayincrease a sulfur utilization rate of a lithium-sulfur battery, andtherefore, may enhance battery life while securing a battery capacityproperty. Accordingly, it is advantageous for batteries including highcapacity and high loading electrodes.

Another preferred embodiment may include 1,3-dioxolane as the firstsolvent, ethylene glycol ethylmethyl ether as the second solvent, andethylene glycol diethyl ether, dipropyl ether or diisobutyl ether as thethird solvent in a volume ratio of 1:1:1 to 1:2:2. This may greatlyenhance Liquid electrolyte stability and thereby significantly improvebattery life. Accordingly, the liquid electrolyte may be suitably usedin batteries operated at high temperatures requiring high liquidelectrolyte stability.

As described above, the liquid electrolyte of the present invention maybe prepared to accord with various characteristics required for abattery by properly selecting a solvent combination.

The liquid electrolyte for a lithium-sulfur battery of the presentinvention includes a lithium salt added to an electrolyte for increasingion conductivity. The lithium salt is not particularly limited in thepresent invention, and those that may be commonly used in lithiumsecondary batteries may be used without limit. Specifically, the lithiumsalt may be one type selected from the group consisting of LiCl, LiBr,LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂, LiC₄BO₈,LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi,(C₂F₅SO₂)₂NLi, (SO₂F)₂NLi, (CF₃SO₂)₃CLi, chloroborane lithium, loweraliphatic carboxylic acid lithium (herein, lower aliphatic means, forexample, aliphatic having 1 to 5 carbon atoms), lithiumtetraphenylborate, lithium imide and combinations thereof, andpreferably, (CF₃SO₂)₂NLi, (C₂F₅SO₂)₂NLi, (SO₂F)₂NLi and the like may beused.

The concentration of the lithium salt may be determined considering ionconductivity and the like, and may preferably be from 0.1 M to 4.0 M, or0.5 M to 2.0 M. When the lithium salt concentration is less than theabove-mentioned range, ion conductivity appropriate for battery drivingis difficult to secure, and when the concentration is greater thanabove-mentioned range, viscosity of the liquid electrolyte increasesdecreasing lithium ion mobility, and battery performance may decline dueto an increase in the decomposition reaction of the lithium salt itself,and therefore, the concentration is properly controlled within theabove-mentioned range.

The liquid electrolyte for a lithium-sulfur battery of the presentinvention may further include an additive having N—O bonds in themolecule. The additive is effective in forming a stable film in alithium electrode and greatly enhancing charge and discharge efficiency.Such an additive may be a nitrate or nitrite-based compound, a nitrocompound and the like. As one example, one or more types selected fromthe group consisting of lithium nitrate, potassium nitrate, cesiumnitrate, barium nitrate, ammonium nitrate, lithium nitrite, potassiumnitrite, cesium nitrite, ammonium nitrite, methyl nitrate, dialkylimidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridiniumnitrate, ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite,octyl nitrite, nitromethane, nitropropane, nitrobutane, nitrobenzene,dinitrobenzene, nitro pyridine, dinitropyridine, nitrotoluene,dinitrotoluene, pyridine N-oxide, alkylpyridine N-oxide, andtetramethylpiperidinyloxyl may be used. According to one example of thepresent invention, lithium nitrate (LiNO₃) may be used.

The additive is used in a range of 0.01% by weight to 10% by weight andpreferably 0.1% by weight to 5% by weight with respect to 100% by weightof the whole liquid electrolyte composition. When the content is lessthan the above-mentioned range, the above-mentioned effects may not besecured, and when the content is greater than the above-mentioned range,resistance may increase due to the film, and therefore, the content isproperly controlled within the above-mentioned range.

As described above, the liquid electrolyte for a lithium-sulfur batteryaccording to the present invention uses a mixed solvent of cyclic etherand linear ether as the solvent for securing liquid electrolytestability, and accordingly, gas generation in a battery may besuppressed during charge and discharge of the battery, and a swellingphenomenon may be improved.

A method for preparing the liquid electrolyte according to the presentinvention is not particularly limited in the present invention, andcommon methods known in the art may be used.

Lithium-Sulfur Battery

A lithium-sulfur battery according to the present invention includes apositive electrode, a negative electrode, a separator providedtherebetween, and a liquid electrolyte, and as the liquid electrolyte,the liquid electrolyte for a lithium-sulfur battery according to thepresent invention is used.

The lithium-sulfur battery according to the present invention hasimproved liquid electrolyte stability and thereby exhibits an excellentlife time property.

The constitution of the positive electrode, the negative electrode andthe separator of the lithium-sulfur battery is not particularly limitedin the present invention, and may follow constitutions known in the art.

Positive Electrode

The positive electrode according to the present invention includes apositive electrode active material formed on a positive electrodecurrent collector.

As the positive electrode current collector, those capable of being usedas a current collector in the art may all be used, and specifically,foamed aluminum, foamed nickel or the like having excellent conductivitymay be preferably used.

The positive electrode active material may include elemental sulfur(S8), sulfur series compounds or mixtures thereof. The sulfur seriescompound may specifically be Li₂S_(n) (n≥≥1), an organosulfur compound,a carbon-sulfur polymer ((C₂S_(x))_(n): x=2.5 to 50, n≥≥2) or the like.These may be used as a composite with a conductor since sulfur materialsalone do not have electrical conductivity.

The conductor may be porous. Accordingly, as the conductor, those havingporosity and conductivity may be used without limit, and for example,carbon-based materials having porosity may be used. As such carbon-basedmaterials, carbon black, graphite, graphene, active carbon, carbon fiberand the like may be used. In addition, metallic fibers such as metalmesh; metallic powders such as copper, silver, nickel and aluminum; ororganic conductive materials such as polyphenylene derivatives may alsobe used. The conductive materials may be used either alone or as amixture.

The positive electrode may further include a binder for binding of thepositive electrode active material and the conductor and for binding onthe current collector. The binder may include a thermoplastic resin or athermosetting resin. For example, polyethylene, polyethylene oxide,polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride(PVDF), styrene-butadiene rubber, a tetrafluoroethylene-perfluoroalkylvinyl ether copolymer, a vinylidene fluoride-hexafluoropropylenecopolymer, a vinylidene fluoride-chlorotrifluoroethylene copolymer, anethylene-tetrafluoroethylene copolymer, a polychlorotrifluoroethylene,vinylidene fluoride-pentafluoro propylene copolymer, apropylene-tetrafluoroethylene copolymer, anethylene-chlorotrifluoroethylene copolymer, a vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene copolymer, a vinylidenefluoride-perfluoromethylvinyl ether-tetrafluoro ethylene copolymer, anethylene-acrylic acid copolymer and the like may be used either alone oras a mixture, however, the binder is not limited thereto, and thosecapable of being used as a binder in the art may all be used.

Such a positive electrode may be prepared using common methods, andspecifically, may be prepared by coating a composition for forming apositive electrode active material layer prepared by mixing a positiveelectrode active material, a conductor and a binder in an organicsolvent on a current collector and drying the result, and selectively,compression molding the result on the current collector for enhancingelectrode density. Herein, as the organic solvent, those capable ofuniformly dispersing the positive electrode active material, the binderand the conductor, and readily evaporating are preferably used.Specifically, acetonitrile, methanol, ethanol, tetrahydrofuran, water,isopropyl alcohol and the like may be included.

Negative Electrode

The negative electrode according to the present invention includes anegative electrode active material formed on a negative electrodecurrent collector.

The negative electrode current collector may specifically be selectedfrom the group consisting of copper, stainless steel, titanium, silver,palladium, nickel, alloys thereof and combinations thereof. Thestainless steel may be surface treated with carbon, nickel, titanium orsilver, and aluminum-cadmium alloys may be used as the alloy. Inaddition thereto, baked carbon, non-conductive polymers of which surfaceis treated with a conductor, conductive polymers or the like may also beused.

As the negative electrode active material, a material capable ofreversibly intercalating or deintercalating lithium ions (Li⁺), amaterial capable of reversibly forming a lithium-containing compound byreacting with lithium ions, lithium metal or a lithium alloy may beused. Examples of the material capable of reversibly intercalating ordeintercalating lithium ions (Li⁺) may include crystalline carbon,amorphous carbon or a mixture thereof. Examples of the material capableof reversibly forming a lithium-containing compound by reacting withlithium ions (Li⁺) may include tin oxide, titanium nitrate or silicon.Examples of the lithium alloy may include alloys of lithium (Li) andmetals selected from the group consisting of sodium (Na), potassium (K),rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium(Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum(Al) and tin (Sn).

The negative electrode may further include a binder for binding of thenegative electrode active material and the conductor and for binding onthe current collector, and specifically, the binder is the same as thebinder of the positive electrode described above.

Separator

A common separator may be provided between the positive electrode andthe negative electrode. The separator is a physical separator having afunction of physically separating electrodes, and those commonly used asa separator may be used without particular limit, and particularly,those having an excellent electrolyte moisture retention ability whilehaving low resistance for ion migration of the liquid electrolyte arepreferred.

In addition, the separator enables lithium ion transfer between thepositive electrode and the negative electrode while separating orinsulating the positive electrode and the negative electrode from eachother. Such a separator may be formed with porous, and non-conductive orinsulating materials. The separator may be an independent member such asa film, or a coating layer added to the positive electrode and/or thenegative electrode.

Specifically, porous polymer films, for example, porous polymer filmsprepared with a polyolefin-based polymer such as an ethylenehomopolymer, a propylene homopolymer, an ethylene/butene copolymer, anethylene/hexene copolymer and an ethylene/methacrylate copolymer may beused either alone or as laminates thereof, or common porous non-wovenfabrics, for example, non-woven fabrics made of high melting point glassfiber or polyethylene terephthalate fiber may be used, however, theseparator is not limited thereto.

The positive electrode, the negative electrode and the separatorincluded in the lithium-sulfur battery may each be prepared using commoncomponents and preparation methods, and although not particularlylimited thereto, appearances of the lithium-sulfur battery may include acylinder-type using a can, a square-type, a pouch-type, a coin-type, andthe like.

Hereinafter, preferred examples are provided in order to illuminate thepresent invention, however, the following examples are for illustrativepurposes only, and it is obvious to those skilled in the art thatvarious changes and modifications may be made within the scope andtechnological ideas of the present invention, and such changes andmodifications also belong to the attached claims.

EXAMPLE Examples 1 and 2 and Comparative Example 1

(1) Preparation of Liquid Electrolyte

Non-aqueous liquid electrolytes of Examples 1 and 2 and ComparativeExample 1 were prepared by adding LiTFSI ((CF₃SO₂)₂NLi) in aconcentration of 1.0 M to a mixed solvent having a composition of thefollowing Table 1, and adding 1% by weight of LiNO₃ thereto based on100% by weight of the liquid electrolyte. Solvents used herein are asfollows.

DOL: 1,3-Dioxolane

DME: 1,2-Dimethoxyethane

EGEME: Ethylene glycol ethylmethyl ether

DPE: Dipropyl ether

TABLE 1 Solvent (Volume Ratio) Lithium Salt Additive Example 1DOL:DME:EGEME 1.0M LiTFSI 1 wt % LiNO₃ (1:1:1) Example 2 DOL:DME:DPE1.0M LiTFSI 1 wt % LiNO₃ (1:1:1) Comparative DOL:DME (1:1) 1.0M LiTFSI 1wt % LiNO₃ Example 1

(2) Manufacture of Lithium-Sulfur Battery

65% by weight of sulfur, 25% by weight of carbon black, and 10% byweight of polyethylene oxide were mixed with acetonitrile to preparepositive electrode active material slurry. The positive electrode activematerial slurry was coated on an aluminum current collector, and theresult was dried to prepare a positive electrode having a size of 30×50mm² and a loading amount of 5 mAh/cm². In addition, lithium metal havinga thickness of 150 μm was employed as a negative electrode.

The prepared positive electrode and the negative electrode were placedto face each other, and a polyethylene separator having a thickness of20 μm was provided therebetween, and the result was filled with theliquid electrolyte prepared in each of the examples and comparativeexamples.

Experimental Example 1 Battery Performance Evaluation

For each of the lithium-sulfur batteries of Examples 1 and 2 andComparative Example 1, specific discharge capacity was measured whileprogressing 20 cycles under the following condition, and the results areshown in FIG. 1.Charge: Rate 0.1 C, voltage 2.8 V, CC/CV (5% current cut at 0.1 C)Discharge: Rate 0.1 C, voltage 1.5 V, CC

As shown in FIG. 1, it was identified that discharge capacity rapidlydecreased after 10 cycles in the battery of Comparative Example 1.However, discharge capacity was stably maintained up to 20 cycles in thebatteries of Example 1 and 2. In addition, a swelling phenomenon wasobserved in the battery of Comparative Example 1 due to gas generatedwith the battery driving, however, a swelling phenomenon was notobserved in the batteries of Examples 1 and 2.

Examples 3 to 6 and Comparative Example 2

(1) Preparation of Liquid Electrolyte

Non-aqueous liquid electrolytes of Examples 3 to 6 and ComparativeExample 2 were prepared by adding LiTFSI ((CF₃SO₂)₂NLi) in aconcentration of 1.0 M to a mixed solvent having a composition of thefollowing Table 2, and adding 1% by weight of LiNO₃ thereto based on100% by weight of the liquid electrolyte. Solvents used herein are asfollows.

DOL: 1,3-Dioxolane

DME: 1,2-Dimethoxyethane

EGEME: Ethylene glycol ethylmethyl ether

EGDEE: Ethylene glycol diethyl ether

DPE: Dipropyl ether

DIBE: Diisobutyl ether

TABLE 2 Lithium Solvent (Volume Ratio) Salt Additive Example 3DOL:EGEME:EGDEE 1.0M 1 wt % (1:1:1) LiTFSI LiNO₃ Example 4 DOL:EGEME:DPE(1:1:1) 1.0M 1 wt % LiTFSI LiNO₃ Example 5 DOL:EGEME:DIBE (1:1:1) 1.0M 1wt % LiTFSI LiNO₃ Example 6 DOL:EGEME:DPE (2:1:1) 1.0M 1 wt % LiTFSILiNO₃ Comparative DOL:DME (1:1) 1.0M 1 wt % Example 2 LiTFSI LiNO₃

(2) Manufacture of Lithium-Sulfur Battery

60% by weight of sulfur, 30% by weight of carbon black, and 10% byweight of polyethylene oxide were mixed with acetonitrile to preparepositive electrode active material slurry. The positive electrode activematerial slurry was coated on an aluminum current collector, and theresult was dried to prepare a positive electrode having a size of 30×50mm² and a loading amount of 5 mAh/cm². In addition, lithium metal havinga thickness of 150 μm was employed as a negative electrode.

The prepared positive electrode and the negative electrode were placedto face each other, and a polyethylene separator having a thickness of20 μm was provided therebetween, and the result was filled with theliquid electrolyte prepared in each of the examples and comparativeexample.

Experimental Example 2 Battery Performance Evaluation

For each of the lithium-sulfur batteries of Examples 3 to 6 andComparative Example 2, specific discharge capacity was measured whilerepeating charge and discharge under the following condition, and theresults are shown in FIG. 2.Charge: Rate 0.1 C, voltage 2.8 V, CC/CV (5% current cut at 0.1 C)Discharge: Rate 0.1 C, voltage 1.5 V, CC

As shown in FIG. 2, it was seen that high initial charge and dischargecapacity was maintained while repeating cycles dozens of times in thelithium-sulfur batteries using the liquid electrolyte of the presentinvention. Meanwhile, it was identified that a battery life enhancingeffect was somewhat low in the battery of Example 6 in which cyclicether occupies 50% of the total solvent volume compared to the batteriesof other examples.

In comparison, the battery of Comparative Example 2 had a tendency ofinitial capacity being greatly reduced after approximately 15 times ofcharge and discharge. Such a result is considered to be due to liquidelectrolyte decomposition occurring during battery driving caused by lowstability of the solvent itself.

From the above-mentioned results, it was seen that the liquidelectrolyte composition of ternary combination of the present inventionenhanced initial charge and discharge capacity retention rate of abattery, and also enhanced battery life when comparing with liquidelectrolytes of existing combinations. In addition, it was seen that theliquid electrolyte of the present invention had a more superior batterylife enhancing effect when a cyclic ether content was less than 40% ofthe total solvent weight.

The invention claimed is:
 1. A liquid electrolyte for a lithium-sulfurbattery comprising: a lithium salt; and a non-aqueous solvent, whereinthe non-aqueous solvent includes i) 1,3-dioxolane (DOL), included in 10%by volume to 40% by volume of a total weight of the non-aqueous solvent;ii) glycol ether represented by the following Chemical Formula 1; andiii) linear ether represented by the following Chemical Formula 2:R¹—O—(CH₂CH₂O)_(x)—R²   [Chemical Formula 1]R³—O—(CH₂CH₂O)_(y)—R⁴   [Chemical Formula 2] in Chemical Formulae 1 and2, R¹ to R⁴ are the same as or different from each other, and eachindependently a C1 to C6 alkyl group, a C6 to C12 aryl group, or a C7 toC13 arylalkyl group; x is an integer of 1 to 4; y is an integer of 0 to4; and the ether of Chemical Formula 1 is different from the ether ofChemical Formula
 2. 2. The liquid electrolyte for a lithium-sulfurbattery of claim 1, wherein the glycol ether is one type selected fromthe group consisting of 1,2-dimethoxyethane, ethylene glycol ethylmethylether, diethylene glycol dimethyl ether, triethylene glycol dimethylether and tetraethylene glycol dimethyl ether.
 3. The liquid electrolytefor a lithium-sulfur battery of claim 1, wherein the linear ether is onetype selected from the group consisting of ethylene glycol ethylmethylether, ethylene glycol diethyl ether, dipropyl ether, diisopropyl ether,dibutyl ether and diisobutyl ether ether.
 4. The liquid electrolyte fora lithium-sulfur battery of claim 1, wherein the glycol ether and thelinear ether are included in a volume ratio of 1:3 to 3:1.
 5. The liquidelectrolyte for a lithium-sulfur battery of claim 1, wherein the lithiumsalt includes one type selected from the group consisting of LiCl, LiBr,LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂, LiC₄BO₈,LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi,(C₂F₅SO₂)₂NLi, (SO₂F)₂NLi, (CF₃SO₂)₃CLi, chloroborane lithium, loweraliphatic carboxylic acid lithium, lithium tetraphenylborate, lithiumimide and combinations thereof.
 6. The liquid electrolyte for alithium-sulfur battery of claim 1, wherein the lithium salt is includedin a concentration of 0.1 M to 4.0 M.
 7. The liquid electrolyte for alithium-sulfur battery of claim 1, further comprising an additive havingN—O bonds in a molecule.
 8. The liquid electrolyte for a lithium-sulfurbattery of claim 7, wherein the additive is one or more types selectedfrom the group consisting of lithium nitrate, potassium nitrate, cesiumnitrate, barium nitrate, ammonium nitrate, lithium nitrite, potassiumnitrite, cesium nitrite, ammonium nitrite, methyl nitrate, dialkylimidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridiniumnitrate, ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite,octyl nitrite, nitromethane, nitropropane, nitrobutane, nitrobenzene,dinitrobenzene, nitro pyridine, dinitropyridine, nitrotoluene,dinitrotoluene, pyridine N-oxide, alkylpyridine N-oxide andtetramethylpiperidinyloxyl.
 9. The liquid electrolyte for alithium-sulfur battery of claim 7, wherein the additive is included in0.01% by weight to 10% by weight with respect to 100% by weight of theliquid electrolyte.
 10. A lithium-sulfur battery comprising the liquidelectrolyte of claim
 1. 11. The liquid electrolyte for a lithium-sulfurbattery of claim 1, wherein the cyclic ether is 1,3-dioxolane (DOL);wherein the glycol ether is selected from the group consisting ofethylene glycol ethylmethyl ether (EGEME), ethylene glycol diethyl ether(EGDEE), and 1,2-dimethoxyethane (DME); and wherein the linear ether isselected from the group consisting of ethylene glycol ethylmethyl ether(EGEME), ethylene glycol diethyl ether (EGDEE), and 1,2-dimethoxyethane(DME), diisobutyl ether (DIBE), and dipropyl ether (DPE).
 12. The liquidelectrolyte for a lithium-sulfur battery of claim 11, wherein thenon-aqueous solvent is selected from the group consisting of:DOL:DME:EGEME; DOL:DME:DPE; DOL:EGEME:EGDEE; DOL:EGEME:DPE; andDOL:EGEME:DIBE.
 13. The liquid electrolyte for a lithium-sulfur batteryof claim 11, wherein the non-aqueous solvent is selected from the groupconsisting of: DOL:DME:EGEME in a volume ratio of 1:1:1; DOL:DME:DPE ina volume ratio of 1:1:1; DOL:EGEME:EGDEE in a volume ratio of 1:1:1;DOL:EGEME:DPE in a volume ratio of 1:1:1; and DOL:EGEME:DIBE in a volumeratio of 1:1:1.
 14. The liquid electrolyte for a lithium-sulfur batteryof claim 11, wherein the lithium salt is LiTFSI.
 15. The liquidelectrolyte for a lithium-sulfur battery of claim 11, further comprisingLiNO₃ as an additive.
 16. A lithium-sulfur battery comprising the liquidelectrolyte of claim 11.